Atomic imaging of oxide surfaces

673

Surface Science 175 (1986) 673-683 North-Holland, Amsterdam

ATOMIC IMAGING OF OXIDE SURFACES I. General features and surface rearrangements David

J. SMITH

Center for Solid State Science and Department Tempe, AZ 85287, USA

OJ Physrcs. Arizona State Unruersity,

L.A. BURSILL School of Physics,

University of Melbourne,

Parkuille,

Victoria 3052, Australta

and D.A. JEFFERSON Department Received

of Physral 24 February

Chemistq,

University of Cambridge,

1986; accepted

for publication

Lensfield

Road, Cambridge,

UK

16 April 1986

Surface profile images from a number of oxides have been obtained using high-resolution electron microscopy. Atomic-level details of the surface topography such as steps and terraces are clearly visible and, with the aid of image viewing and recording systems, cation redistributions on oxide surfaces can be followed in real time. Electron irradiation is shown to have different effects on the surfaces of different oxides. Transition metal oxides appear to desorb oxygen, effectively leading to “metallisation” of the surface, whereas holes are etched in alumina and nickel oxide. and surface rearrangements are the predominant effects seen in rare earth oxides.

1. Introduction In recent high-resolution electron microscope (HREM) studies it has been demonstrated that the profile imaging technique can provide atomic scale information about the morphology of semiconductor [l-3], metal [4-131 and oxide [14-191 surfaces. Most effort has been concentrated on small gold particles [4,9-131 and extended gold foils [5-81. This is partly because these have surfaces which are comparatively inert and which are easily kept clean during irradiation by the electron beam in the moderate (10-6-10~7 Torr) vacuum of the microscope. Moreover, except for possible carbon monolayers [5,20], atoms of other elements are not present and this makes the interpretation of surface images relatively straightforward since it is not necessary to consider the variety of bulk lattice terminations which can occur in compound 0039-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

materials even when they do not develop different surface structures (which is often the case). Nevertheless, Hutchison and Briscoe [14,15] were able to characterise the surface features of spinet catalyst particles, including modifications which had occurred as a result of catalytic reaction. The reduction, and metallisation, which occurs on the clean surfaces of some oxide particles due to various electron-stimulated desorption (ESD) processes have also been monitored at the atomic level using profile imaging [l&19]. It is the objective of this paper to demonstrate that a wide range of morphological information on the atomic scale about the surfaces of oxides is available through judicious use of the profile imaging technique, even under circumstances when it is not possible to establish unequivocally the exact nature of the bulk lattice terminations. In subsequent papers, we will describe in more detail the surface features of both anion-deficient [21] and anion-excess [22] oxides and some electron-stimulated surface reactions which occur as a result of electron irradiation in several nonstoichiometric and doped tungsten oxides. in various types of alumina, and in rutile.

2. Experimental

details

The oxide specimens described here were prepared for observation in the normal way by grinding under alcohol with a mortar and pestle and allowing a drop of the suspension to dry on a microscope specimen grid which had been pre-coated with a holey carbon support film. Despite care with preparation. present; thin layers of surface contamination (- l-2 nm) were sometimes however. in most cases, these layers were usually removed during the initial observation period. Two electron microscopes were used: the Cambridge University HREM. operated at 500 kV, and the JEM-4000EX at Arizona State University, which was operated primarily at 400 kV. Both instruments were equipped with double-tilt specimen holders in order that crystal zone axes could be accurately aligned with the incident beam direction, and both had image pickup and viewing systems so that direct real-time observation and recording of dynamic events was possible. At the optimum objective lens defocus, both microscopes had interpretable, or “point-to-point” resolutions of about 0.17-0.18 nm, and individual cation columns could generally be resolved for suitably thin specimens in several low index crystal projections. the cation arrangements and rearrangeUnder these operating conditions, ments along the crystal edges could be clearly seen. Note that image simulations have been carried out for several of the examples shown here and these confirmed the one-to-one correspondence between image features and the crystal structure for images recorded close to optimum defocus. These computations, which are not described here, justify direct interpretation of the surface profile images in terms of the cation column positions, although they

D.J. Smith et al. / Oxide surfaces. I. General features

615

also indicate that the presence, or absence, of oxygen atoms along the surface usually cannot be ascertained from the profile image.

3. Results 3.1. Bismuth-tungsten-bronzes Our primary motivation for studying bismuth-tungsten-bronzes was to characterize their oxidation and decomposition processes [23]. However, our initial observations indicated that preparation of these materials generally resulted in surfaces effectively free of contamination so that the surface morphology of the bronze could also be characterized in profile at the atomic level. The 500 kV micrographs shown in fig. 1 come from a sample of nominal composition Bi,,,WO,, with th’1s particular crystal oriented in the [loo] projection. With this approximate stoichiometry, it is found [23] that the Bi atoms occupy the tunnel sites in the strips of hexagonal tungsten bronze (HTB) phase (arrowed in fig. la) which are separated by slabs of WO,. Electron irradiation leads to a depletion of Bi atoms from these tunnel sites which is apparent in electron micrographs, such as those in fig. 1, from the variations

Fig. 1. (a) High-resolution electron micrograph recorded at 500 kV showing the edge of a bism huth-tungsten-bronze crystal with some surface contamination present. Note the variatic ,ns of inter lsity along the (arrowed) HTB layers indicating variable occupancy of the Bi atom co lumn sites. (b) Surface profile of bismuth-tungsten-bronze crystal following removal of surface cant amination. Note columns (arrowed) which are presumed to be Bi atoms out of register with the WO, pseudocubic lattice.

D.J. Smiih et al. / Oxide surfaces

Fig. 2. (a) Surface profile image of bismuth-tungsten-bronze crystal in [OIO] projection soon after onset of irradiation. (b) Surface profile image of same crystal edge, recorded after continuous electron irradiation for about 15 min. showing rounding of facetted edges at A and B and redistribution of atom columns along the edge.

of intensity which start to appear along these HTB strips during observation. At the same time, there is a gradual buildup of intensity along the surface at positions which are not usually commensurate with the bulk lattice: examples are arrowed in fig. lb. This contrast presumably originates from the displaced Bi atoms which become aligned in columns along the beam direction after diffusion to the surface. Other events are observed to occur at and near the surfaces of these bismuth-tungsten-bronzes during extended periods of irradiation. As well as the redistribution of Bi atoms along the surfaces, there is a tendency, as shown in figs. 2a and 2b, to round off, or flatten, any sharp corners which result from the initial fracture of the crystals during preparation. Moreover, oxygen is also

Fig. 3. Crystal accommodated

of bismuth tungstate, of nominal composition Bi,,,,,WO,. Loss of oxygen by precipitation of crystallographtc shear defects (P) and the “metallisation” near-surface layers (see examples arrowed).

is of

D.J. Smith et al. / Oxde surfaces.

Fig. 4. Surface profile image of a chromia-doped rutile crystal, nominal composition Ti(Cr)O, qz (CS defects arrowed in bulk). Note lower intensity and general lack of order of immediate surface layer.

lost from the crystals and this loss of stoichiometry is manifested in the precipitation of crystallographic shear planes within the crystal and the “metallisation” of the near-surface layers *. Both of these processes have been initiated in the region of crystal shown in fig. 3. Other examples of metallisation are shown below. 3.2. Chromia-doped

rutile

The loss in stoichiometry which results from doping rutile (TiO,) with chromia (Cr,O,) leads to the precipitation of extended defects, commonly known as crystallographic shear planes (CSP). These CS defects are the diagonal lines arrowed on the crystal shown in fig. 4, which is from a sample of nominal composition Ti(Cr)O,,,, in a [Oil] projection. Although the CS defects are regions of high Cr concentration, it is clear that the intensity of the atomic columns along the defect is not noticeably different from those in the remainder (which is not unexpected given the closeness of Ti and Cr in the Periodic Table). Hence, there is no direct or even indirect way of differentiating between the atomic species at the surface, as was the case for figs. 1 and 2. Nevertheless, from the surface profile image it is still possible to state that the surface once again seems to be free of contamination, and the dispositions of atomic columns right up to, and along, the surface are clearly visible in the form of steps and terraces. 3.3. Vanadium-niobium

oxide

The images shown in figs. 5a and 5b are from the same region of a crystal of VNb,O,, in a [OOl] projection but they were recorded with a time sep* The phenomenon of “metallisation” which has been observed in CdS [3] and in several other oxides [l&19] as well as those shown here, is believed to originate from preferential electronstimulated desorption (ESD) of the anion species.

D.J. Smith et al. / Oxide surfaces. I. Generulfeutuws

Fig. 5. Region of vanadium niobate crystal. composition VNb,C&, showing the “metallisation” near-surface layers due to electron-stimulated desorption of oxygen, following the removal surface contamination: (b) was recorded about 45 min later than (a).

of of

aration of about 45 min. In fig. 5a, a thin carbon contamination layer is still present along the edge of the crystal whereas. in fig. 5b. this layer has been etched away and the crystal structure along the edge has also changed. Finely spaced lattice fringes are now visible to a depth of 1.5-2 nm from the crystal edge. These fringes. which have spacings of about 0.2-0.21 nm, appear to form a square array which is closely aligned with the existing major axes of the bulk crystal. A similar, apparently epitaxial. development has been documented previously along the surfaces of Ti,Nb,,O,, crystals [19]. As described in detail elsewhere [l&19]. these examples are thought to be cases of the ESD of oxygen from near-surface sites effectively leaving behind a metal-rich surface layer. 3.4. Molybdenum oxide In the niobium and tungsten oxides, the metallisation process did not usually appear to take place until the carbon layer was removed. This did not happen for surfaces of the molybdenum (tantalum) oxide, (MO, Ta),O,,. As

619

D.J. Smith et al. / Oxide surfaces. I. General features

Fig. 6. Edge of molybdenum-tantalum oxide crystal showing the onset regions arrowed) despite the presence of surface contamination

of metallisation layer.

shown in fig. 6. packets of fine fringes are already along the crystal edge despite the presence of carbon.

at several

visible

(see

places

3.5. Alumina In the case of a potential ruby laser material, based on a chromium-doped alumina, a totally different phenomenom occurred as a result of electron irradiation. As shown by the low magnification image in fig. 7, this material started to develop a “patchwork quilt” appearance. This was first thought to be due to knock-on atomic displacement by the incident electron beam. since the electron energy was above the threshold for both Al and 0 displacement [24]. However, observation at 100 kV again showed that, under intense electron irradiation (lo-30 A/cm’), the same appearance still developed even though the electron energy was well below the displacement threshold for

Fig. 7. Region of ruby-alumina showing the patchwork appearance caused Note also the development of surface facets.

by electron

irradiation.

Fig. 8. Regiqn of ruby--alumina are seen at 100 kV). showing development

following extended irradiation at 400 kV (although similar effects the complete performation of holes through the material and the of anomalous dark contrast along certain facets.

either atomic species. Further irradiation led eventually to the formation of holes right through the crystals and, as shown in fig. 8, these holes were usually facetted along (0006) planes with an anomalously dark and structureless appearance. This latter effect is thought to be due either to the accumulation of charge on these surfaces or to the development of a surface superstructure which is inclined to the incident beam direction. A detailed description of related surface effects for ruby, sapphire and other alumina crystals will be given elsewhere [25]. 3.6. Nickel oxide

Examination of NiO crystals at 400 kV also led to the development of holes which were again often facetted. However, unlike the alumina samples, small crystals were frequently formed which had orientations, and sometimes lattice spacings, which were different to those of the original lattice. Examples have been arrowed in the high-resolution profile image shown in fig. 9. By careful cross-calibration of lattice spacings, using the NiO(200) fringes of 0.21 nm as an internal reference, it was possible to identify these packets as being crystals of NiO in the [110] orientation rather than (100]. 3.7. Uranium oxide Profile imaging of UO, and U,O, crystals revealed surfaces which were remarkably free of contamination. Surfacefteps and ledges were clearly visible and observation with the image pickup system indicated that considerable atomic diffusion also occurred along some surfaces. leading eventually to

D.J. Smith et (11. / Ox&

surfaces.

I. General features

6X1

Fig. 9. Crystal of nickel oxide originally in a [OOl] projection showing the formation of holes and the development of structural disorder due to electron irradiation. Small crystals in [110] projections are visible at A, B and C.

substantial rearrangements of the particular profile. For an example of these changes, comparison should be made between the two images of a UO, crystal in a [IlO] projection shown in fig. 10 which were recorded with a time difference of about 5 min. Furthermore, a detailed examination of the surfaces of these oxides [22] indicates that different surfaces have different responses with respect to both electron irradiation and to in situ annealing treatments, a

Fig. 10. Surface

profile images from a uranium crystal in a [110] projection, time difference showing movement of atomic columns (arrowed) between exposures.

5 min.

result which is these surfaces. highly resistant described above

not unexpected given the differences in binding energies of Finally, note that the surfaces of these oxides proved to be to the various reduction, metallisation and irradiation effects for other materials.

4. Discussion From the selection of atomic-resolution images presented here, it is clear that the technique of surface profile imaging represents an invaluable means for characterising directly the local morphology of oxide surfaces at a level generally unapproachable by other techniques. Whilst it needs to be remembered that the details visible in a profile image originate from a projection of the surface structure in the beam direction, i.e. no relative depth discrimination is possible, features such as steps, terraces and surface defects are unmistakable. Moreover. with the aid of real-time viewing and recording. atomic rearrangements are easily followed and differences in atomic mobilities on different surfaces for example can be readily established. The technique has great potential for studying dynamic processes occurring on oxide surfaces such as atomic diffusion, reconstruction and nucleation and for documenting the changes which result from catalytic reactions. It will be more difficult and time-consuming to obtain quantitative information from profile images. For example. certain oxide surfaces are liable to reconstruct into lattice structures unlike the bulk. other surface layers may contract. or perhaps expand. Accurate information about the atomic positions at such modified surfaces is only obtainable after careful analysis based on detailed comparisons between experimental and simulated images. This is because the finer details of all high-resolution electron micrographs are known to be highly sensitive to the particular imaging conditions of the electron microscope, such as the energy spread of the electron beam, the objective lens defocus and astigmatism, and the incident beam misalignment. Surface profile images are also strongly influenced by edge diffraction effects. Moreover, errors in simulated images can arise unless appropriate precautions are taken these restrictions, it appears that the positions of [20]. Notwithstanding individual atomic columns can sometime be reliably located to within about 0.02-0.03 nm [20,26]. It is important to realize, as demonstrated in several of the examples above, that the high energy electron beam of the HREM will almost certainly modify the oxide surface during the imaging process. Depending on such factors as the energy, the current density and the total dose of the electron beam incident on the sample, the surface cleanliness and the microscope vacuum, and finally the characteristics of the sample such as the nature of its bonding (ionic or covalent), its energy of formation and the binding energies of different surfaces, then different events take place. For example, surface reduction or

D.J. Smith et al. / Oxide surfaces. I. Generul features

683

metallisation due to ESD processes occur for some transition metal oxides [18,19] but not for rare earth binary oxides [21,22]. Atomic rearrangements can lead to changes in profile, particularly for the non-equilibrated surfaces of fractured crystals. The strong likelihood of such beam-induced modifications needs to be recognized during any HREM surface study, just as it is already accepted when other techniques involving energetic or ionising radiation are used [27]. Acknowledgements

This work has used facilities at the Cambridge University High Resolution Electron Microscope supported by the Science and Engineering Research Council (UK) and at the Arizona State University Facility for High Resolution Electron Microscopy, within the Center for Solid State Science. provided by NSF Grant DMR-830871 and supported by NSF Grant DMR-8306501. Support from the Arizona State University Research Fund is also acknowledged. We are grateful to Professor L. Kihlborg, Dr. Peng Ju Lin, Dr. K.L. Merkle and Professor C.N.R. Rao for the provision of samples. References [l] [2] [3] [4] [5] (61 [7] ]X] (91 [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] (251 [26] [27]

R. Sinclair, T. Yamashita and F.A. Ponce, Nature 290 (1982) 286. J.M. Gibson, M.L. McDonald and F.C. Unterwald, Phys. Rev. Letters, 55 (1985) 1765. D.J. Ehrlich and D.J. Smith. Appl. Phys. Letters. in press. L.D. Marks and D.J. Smith, Nature 303 (1983) 316. L.D. Marks and D.J. Smith, Surface Sci. 143 (1984) 495. D.J. Smith and L.D. Marks, Ultramicroscopy 16 (1985) 101. L.D. Marks and D.J. Smith. Surface Sci. 157 (1985) L367. D.J. Smith and L.D. Marks, Surface Sci., submitted. L.R. Wallenberg. J.-O. Bovin and G. Schmid, Surface Sci. 156 (1985) 256. J.-O. Bovin, L.R. Wallenberg and D.J. Smith. Nature 317 (1985) 47. L.R. Wallenberg, J.-O. Bovin and D.J. Smith, Naturwissenschaften 72 (1985) 539. D.J. Smith, A.K. Petford. J.-O. Bovin and L.R. Wallenberg. Science, in press. S. lijima and T. Ichihashi, Japan. J. Appl. Phys. 24 (1985) L125. N.A. Briscoe and J.L. Hutchison. Inst. Phys. Conf. Ser. 68 (1984) 249. J.L. Hutchison and N.A. Briscoe, Ultramicroscopy 18 (1985) 435. S. Iijima and M. Ichikawa, J. Catalysis 94 (1985) 313. C.E. Warble, Ultramicroscopy 15 (1984) 301. A.K. Petford, L.D. Marks and M. O’Keeffe, Surface Sci. 172 (1986) 496. D.J. Smith and L.A. Bursill, Ultramicroscopy 17 (1985) 387. L.D. Marks. Surface Sci. 139 (1984) 281. Z.C. Kang. D.J. Smith and L. Eyring, Surface Sci. 175 (1986) 684. D.J. Smith, L.A. Bursill and Peng Ju Lin, submitted. D.A. Jefferson. M.K. Uppal and D.J. Smith, J. Solid State Chem. 53 (1984) 101. A.Y. Stathopoulos and G.P. Pells, Phil. Msg. A47 (1983) 381. L.A. Bursill. D.J. Smith and Peng Ju Lin, in preparation. W.O. Saxton and D.J. Smith, Ultramicroscopy 18 (1985) 39. D. Menzel, Ultramicroscopy 14 (1983) 175.